wasm: controlled screen destruction

Freeing OpenGL resources requires a current context,
which (on wasm) requires a screen.

Add a destroy() functions to QWasmScreen, QWasmCompositor, QWasmWindow,
and QWasmBackingStore which facilitates OpenGL cleanup before we start
deleting screen objects.

Task-number: QTBUG-75463
Change-Id: I9954b536416b9147965c74459ccad838d1578778
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/plugins/platforms/wasm/qwasmbackingstore.cpp

@@ -55,6 +55,12 @@ QWasmBackingStore::~QWasmBackingStore()
 {
 }
 
+void QWasmBackingStore::destroy()
+{
+    if (m_texture->isCreated())
+        m_texture->destroy();
+}
+
 QPaintDevice *QWasmBackingStore::paintDevice()
 {
     return &m_image;

src/plugins/platforms/wasm/qwasmbackingstore.h

@@ -44,6 +44,7 @@ class QWasmBackingStore : public QPlatformBackingStore
 public:
     QWasmBackingStore(QWasmCompositor *compositor, QWindow *window);
     ~QWasmBackingStore();
+    void destroy();
 
     QPaintDevice *paintDevice() override;
 

src/plugins/platforms/wasm/qwasmcompositor.cpp

@@ -37,6 +37,7 @@
 #include <QtGui/qopenglcontext.h>
 #include <QtGui/qopenglfunctions.h>
 #include <QtGui/qopengltextureblitter.h>
+#include <QtGui/qoffscreensurface.h>
 #include <QtGui/qpainter.h>
 #include <private/qpixmapcache_p.h>
 
@@ -71,6 +72,28 @@ QWasmCompositor::QWasmCompositor(QWasmScreen *screen)
 QWasmCompositor::~QWasmCompositor()
 {
     delete m_frameBuffer;
+    destroy();
+}
+
+void QWasmCompositor::destroy()
+{
+    // Destroy OpenGL resources. This is done here in a separate function
+    // which can be called while screen() still returns a valid screen
+    // (which it might not, during destruction). A valid QScreen is
+    // a requirement for QOffscreenSurface on Wasm since the native
+    // context is tied to a single canvas.
+    if (m_context) {
+        QOffscreenSurface offScreenSurface(screen()->screen());
+        offScreenSurface.setFormat(m_context->format());
+        offScreenSurface.create();
+        m_context->makeCurrent(&offScreenSurface);
+        for (QWasmWindow *window : m_windowStack)
+            window->destroy();
+        m_blitter.reset(nullptr);
+        m_context.reset(nullptr);
+    }
+
+    m_isEnabled = false; // prevent frame() from creating a new m_context
 }
 
 void QWasmCompositor::setEnabled(bool enabled)
@@ -653,7 +676,7 @@ void QWasmCompositor::frame()
 
     m_needComposit = false;
 
-    if (m_windowStack.empty() || !screen())
+    if (!m_isEnabled || m_windowStack.empty() || !screen())
         return;
 
     QWasmWindow *someWindow = nullptr;
@@ -676,7 +699,9 @@ void QWasmCompositor::frame()
         m_context->create();
     }
 
-    m_context->makeCurrent(someWindow->window());
+    bool ok = m_context->makeCurrent(someWindow->window());
+    if (!ok)
+        return;
 
     if (!m_blitter->isCreated())
         m_blitter->create();

src/plugins/platforms/wasm/qwasmcompositor.h

@@ -64,6 +64,7 @@ class QWasmCompositor : public QObject
 public:
     QWasmCompositor(QWasmScreen *screen);
     ~QWasmCompositor();
+    void destroy();
 
     enum QWasmSubControl {
         SC_None =                  0x00000000,

src/plugins/platforms/wasm/qwasmintegration.cpp

@@ -258,7 +258,9 @@ void QWasmIntegration::addScreen(const QString &canvasId)
 
 void QWasmIntegration::removeScreen(const QString &canvasId)
 {
-    QWindowSystemInterface::handleScreenRemoved(m_screens.take(canvasId));
+    QWasmScreen *exScreen = m_screens.take(canvasId);
+    exScreen->destroy(); // clean up before deleting the screen
+    QWindowSystemInterface::handleScreenRemoved(exScreen);
 }
 
 void QWasmIntegration::resizeScreen(const QString &canvasId)

src/plugins/platforms/wasm/qwasmscreen.cpp

@@ -57,7 +57,12 @@ QWasmScreen::QWasmScreen(const QString &canvasId)
 
 QWasmScreen::~QWasmScreen()
 {
+    destroy();
+}
 
+void QWasmScreen::destroy()
+{
+    m_compositor->destroy();
 }
 
 QWasmScreen *QWasmScreen::get(QPlatformScreen *screen)

src/plugins/platforms/wasm/qwasmscreen.h

@@ -52,6 +52,7 @@ class QWasmScreen : public QObject, public QPlatformScreen
 public:
     QWasmScreen(const QString &canvasId);
     ~QWasmScreen();
+    void destroy();
 
     static QWasmScreen *get(QPlatformScreen *screen);
     static QWasmScreen *get(QScreen *screen);

src/plugins/platforms/wasm/qwasmwindow.cpp

@@ -65,6 +65,12 @@ QWasmWindow::~QWasmWindow()
     m_compositor->removeWindow(this);
 }
 
+void QWasmWindow::destroy()
+{
+    if (m_backingStore)
+        m_backingStore->destroy();
+}
+
 void QWasmWindow::initialize()
 {
     QRect rect = windowGeometry();

src/plugins/platforms/wasm/qwasmwindow.h

@@ -58,6 +58,7 @@ public:
 
     QWasmWindow(QWindow *w, QWasmCompositor *compositor, QWasmBackingStore *backingStore);
     ~QWasmWindow();
+    void destroy();
 
     void initialize() override;